What is the angle of the the mirror?

1. The problem statement, all variables and given/known data
There is an intriguing aspect to Vermerr's "The Music" painting. If you notice, in the mirror there is a reflection of the woman and above that there is a reflection of what appears to be the feet of an easel. Upon closer inspection you can see the base of the back wall reflected in the mirror. The intersection of the back wal and floor in the mirror is marked on the outline in the image. We can use this as a second method to determine the length of the room. But the question is, what is the angle from the wall at which the mirror hangs?
Picture given: http://www.freeimagehosting.net/image.php?2e866c251b.jpg

Do you see the dark shadow and lighter shadow at the paintings and the open chests right side ?? The long and lighter shadow is caused by the first set of windows. The windows at back can reach where the front ones cannot so that makes it lighter. As you can see there is a darker shadow also. I suppose that the room consists of 2 sets of 4 windows according to dark and light shadows and my predictions. Because if there were more sets of windows then there would be dark, light and lighter shadows. So by calculating the lenght of the dark shadows (only dark ones) at the time where they are equal by lenght, it shows that, at that point they are equally far from the front wall. Aquire the lenghts of painting and chest, desk or whatever it is, you can calculate the angle. but maybe the colomn in the middle is reflection of room. I mean if there is 3 metres front the middle colomn there will be 3 at back so lenght is 3+3+lenght of colomn.

What confuses me is how to get the angle... I think it may have something to do with optics on mirrors... I know the angle should be 15 degrees or about that because I measured it from a to scale diagram, but I need to actually find it...

since it's a painting the "actuality" is not even concerned. But try the dark shadow thing i said right up.

edit: whatever you do with the painting, you will get a ratio of lenghts, so how can you predict the actuall? The lady may be 1,50m or 1,80m and the rooms lenght would slightly change. I suppose you need something like a tiles lengh.